Secondary batteries are often provided to users in the form of rechargeable battery packs which can be repeatedly used. In particular, in order to protect users and electronic appliances, lithium ion secondary batteries having a high volumetric energy density typically include several protective circuits incorporated in battery packs for over-charging protection and over-discharging protection to interrupt the output of the battery pack under predetermined conditions.
Some of these protective elements use an FET switch incorporated in a battery pack to turn ON/OFF the output, for over-charging protection or over-discharging protection of the battery pack. However, even in the cases of the FET switch being short-circuited and damaged for some reason, a large current caused by a surge such as lighting momentarily flows, and an abnormally decreased output voltage or an excessively high voltage occurs in an aged battery cell, the battery pack or the electronic appliance should prevent accidents including fire, among others. For this reason, a protective element is used having a fuse which interrupts a current path in accordance with an external signal so as to safely interrupt the output of the battery cell under these possible abnormalities.
As shown in FIG. 15 (A) and FIG. 15 (B), in such a protective element 80 of a protective circuit for lithium ion secondary batteries, a meltable conductor 83 is connected between a first and second electrodes 81, 82 as a part of a current path and the meltable conductor 83 on the current path is blown by self-heating caused by an overcurrent or by a heat-generating resistor 84 provided within the protective element 80. In such a protective element 80, the molten meltable conductor 83, now in a liquid form, gathers on the first and second electrodes 81, 82 to interrupt the current path.
Additionally, in such a protective element 80 as illustrated in FIG. 15, in general, a Pb containing high melting point solder having a melting point of 300° C. or more is used as the meltable conductor 83 so that melting does not occur during mounting by reflow solder bonding. Moreover, because heating the meltable conductor 83 causes oxidation which inhibits blowout, a flux body 85 is laminated thereon in order to remove oxide film generated on the meltable conductor 83 and improve wettability of the meltable conductor 83.